Collimator structure for an imaging system

ABSTRACT

Methods and systems are provided for reducing aliasing artifacts in images generated in an imaging system, such as a computed tomography (CT) imaging systems. In one embodiment, a method comprises receiving a filtered signal generated by passing a primary signal through a collimator having a non-rectangular transition profile, the collimator positioned between an object and a detector of an imaging system, and generating an image based on the filtered signal received at the detector, the primary signal including radiation from a source of the imaging system attenuated by the object placed between the source and the detector. In this way, by filtering the signals at the collimator before reaching the detector, aliasing artifacts in the images may be reduced.

FIELD

Embodiments of the subject matter disclosed herein relate tonon-invasive diagnostic imaging, and in some examples, to a collimatorof a computed tomography (CT) imaging system that attenuates radiationsignals in a non-uniform manner.

BACKGROUND

Non-invasive imaging technologies allow images of the internalstructures of an object (e.g., patient) to be obtained withoutperforming an invasive procedure on the object. In particular,technologies such as computed tomography (CT) use various physicalprinciples, such as the differential transmission of x-rays through thetarget volume, to acquire image data and to construct tomographic images(e.g., three-dimensional representations of the interior of the humanbody or of other imaged structures).

In CT imaging systems, a source projects a beam (e.g., x-ray beam),which passes through an object being imaged, and impinges upon an arrayof detectors. As such, the beam is attenuated by the object, and theintensity of the beam received at the array of detectors depends on theamount by which the beam is attenuated by the object. The attenuatedbeam is then used to reconstruct the tomographic images.

Typically, the CT imaging systems include a pre-patient collimatorpositioned proximate to the source that defines the profile of the beampassing through the object. In addition, the CT imaging systems includea post-patient collimator positioned in front of the array of detectorsto shield the detectors from scattered radiation.

Generally, the post-patient collimator used in CT imaging systemsincludes a rectangular spatial profile. In some examples, thepost-patient collimator may be formed with a high attenuation materialsuch as tungsten or lead. Specifically, the post-patient collimator mayinclude open regions (or low attenuation regions) in between highattenuation regions forming a grid-like pattern. As such, a portion ofthe beam passes through the open regions and reaches the detectors, andthe remaining portion of the beam is blocked by the high attenuationregions. In this way, the post-patient collimator may control the amountof radiation reaching the detector, reduce unwanted scattered radiationfrom reaching the detector, and thereby reduce noise in the systems.However, such post-collimators may lead to a spatial domain response ofthe detector that may cause high frequency components in the signal towrap around and form aliasing artifacts in the final CT images thatreduces the quality of the CT images.

BRIEF DESCRIPTION

In one embodiment, a method comprises receiving a filtered signalgenerated by passing a primary signal through a collimator having anon-rectangular transition profile, the collimator positioned between anobject and a detector of an imaging system, and generating an imagebased on the filtered signal received at the detector, the primarysignal including radiation from a source of the imaging systemattenuated by the object placed between the source and the detector.Herein, the non-rectangular transition profile of the collimator maymodulate the primary signal before the signal reaches the detector. Assuch, modulating the primary signal results in filtering the analogsignals in a spatial domain of the collimator and reducing highfrequency components of the primary signal, even before reaching thedetector. In this way, by pre-filtering the signals and reducing highfrequency components of the signals, aliasing artifacts in the CT imagesmay be reduced.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a pictorial view of an imaging system according to anembodiment of the invention;

FIG. 2 shows a block schematic diagram of an exemplary imaging systemincluding a post-patient collimator, according to an embodiment of theinvention;

FIG. 3A shows the post-patient collimator having a rectangulartransition profile, according to an embodiment of the invention;

FIG. 3B shows the post-patient having an anti-aliasing shape resultingin a modulated transition profile, according to an embodiment of theinvention;

FIG. 4A shows a spatial domain response of a detector of the imagingsystem with the post-patient collimator of FIG. 3A, according to anembodiment of the invention;

FIG. 4B shows a spatial domain response of a detector of the imagingsystem with the post-patient collimator of FIG. 3B having theanti-aliasing shape, according to an embodiment of the invention;

FIG. 5 shows a frequency domain response of the detector of the imagingsystem, according to an embodiment of the invention;

FIG. 6 shows an example CT image with aliasing artifacts, according toan embodiment of the invention;

FIG. 7 shows a collimator plate having an anti-aliasing shape, accordingto an embodiment of the invention;

FIG. 8A shows a magnified view of the collimator plate, according to anembodiment of the invention;

FIG. 8B shows a transmission profile of the collimator plate, accordingto an embodiment of the invention;

FIG. 9 shows a modulation transfer function of the detector having thecollimator plate, according to an embodiment of the invention; and

FIG. 10 shows a high-level flow chart illustrating an example method forreducing aliasing artifacts in the imaging system using collimatorshaving smoother transition profiles.

DETAILED DESCRIPTION

The following description relates to various embodiments of medicalimaging systems. In particular, methods and systems are provided forreducing aliasing artifacts in the imaging systems. An example of acomputed tomography (CT) imaging system that may be used to acquireimages processed in accordance with the present techniques is providedin FIGS. 1 and 2. The CT imaging system may include a pre-patientcollimator positioned in between a source and a patient that is beingscanned, and further includes a post-patient collimator or anti-scattercollimator positioned in front of a detector (FIG. 2). While thepre-patient collimator adjusts the width of a source beam, thepost-patient collimator controls the width of an attenuated beam afterpassing through the patient before reaching the detector. Typically, thepost-patient collimator includes a rectangular transition profile asshown in FIG. 3A, having low attenuation regions or plates juxtaposed tohigh attenuation regions or plates. However, the rectangular profile ofthe post-patient collimator may lead to aliasing artifacts in the CTimage. Herein, the aliasing artifacts are caused due to higherfrequencies in the signal wrapping around and contaminating the signaldetected by the detector as shown in an example CT image in FIG. 6.

According to embodiments disclosed herein, the shape of the post-patientcollimator may be adjusted so that the aliasing artifacts may be reducedin order to generate a smoother transition profile. The smoothertransition profiles lead to a reduction in the higher frequency signalsthat otherwise wrap around and generate the aliasing artifacts in theimages. In one example, the post-patient collimator may includebell-shaped regions, as shown in FIG. 3B. A spatial domain response ofthe detector of the imaging system with the post-patient collimatorhaving a bell-shaped structure is shown in FIG. 4B, and as such, thisresults in a reduced aliasing effect, as observed in the detectorfrequency response curve (FIG. 5). In another example, the anti-aliasingshape may be incorporated in a collimator plate coupled to the detectoras shown in FIG. 7. An exploded view of the collimator plate is shown inFIG. 8A and a transmission profile of the collimator plate is shown inFIG. 8B. A modulated transfer function (MTF) of the detector (FIG. 9)shows a reduction in the higher frequency components detected by thedetector when the anti-aliasing collimator plate is used. A method forreducing aliasing artifacts by pre-filtering the signals using theanti-aliasing shape of the collimator is shown in FIG. 10. It may beappreciated that the pre-filtering is performed in the analog domain,before the data is digitized. In this way, the anti-aliasing collimatormay serve as analog pre-filter that reduces aliasing artifacts withoutperforming adjustments such as adjusting a focal spot of the source oradjusting a shape of the detector cell, for example.

Though a CT system is described by way of example, it should beunderstood that the present techniques may also be useful when appliedto images acquired using other imaging modalities, such astomosynthesis, and so forth. The present discussion of a CT imagingmodality is provided merely as an example of one suitable imagingmodality. Other imaging modalities that may be use the presenttechniques include diagnostic radiography, tomosynthesis, cone beam CT,and other modalities that utilize collimators. Further, the principlesdescribed herein may also apply to ultrasound if the “collimator” isused to attenuate sound.

FIG. 1 illustrates an exemplary CT system 100 configured to allow fastand iterative image reconstruction. Particularly, the CT system 100 isconfigured to image a subject such as a patient, an inanimate object,one or more manufactured parts, and/or foreign objects such as dentalimplants, stents, and/or contrast agents present within the body. In oneembodiment, the CT system 100 includes a gantry 102, which in turn, mayfurther include at least one x-ray radiation source 104 configured toproject a beam of x-ray radiation 106 for use in imaging the patient.Specifically, the radiation source 104 is configured to proj ect thex-rays 106 towards a detector array 108 positioned on the opposite sideof the gantry 102. Although FIG. 1 depicts only a single radiationsource 104, in certain embodiments, multiple radiation sources may beemployed to project a plurality of x-rays 106 for acquiring projectiondata corresponding to the patient at different energy levels.

In certain embodiments, the CT system 100 further includes an imageprocessing unit 110 configured to reconstruct images of a target volumeof the patient using an iterative or analytic image reconstructionmethod. For example, the image processing unit 110 may use an analyticimage reconstruction approach such as filtered backprojection (FBP) toreconstruct images of a target volume of the patient. As anotherexample, the image processing unit 110 may use an iterative imagereconstruction approach such as advanced statistical iterativereconstruction (ASIR), conjugate gradient (CG), maximum likelihoodexpectation maximization (MLEM), model-based iterative reconstruction(MBIR), and so on to reconstruct images of a target volume of thepatient.

FIG. 2 illustrates an exemplary imaging system 200 similar to the CTsystem 100 of FIG. 1. In one embodiment, the system 200 includes thedetector array 108 (see FIG. 1). The detector array 108 further includesa plurality of detector elements 202 that together sense the x-ray beams106 (see FIG. 1) that pass through a subject 204 such as a patient toacquire corresponding projection data. Accordingly, in one embodiment,the detector array 108 is fabricated in a multi-slice configurationincluding the plurality of rows of cells or detector elements 202. Insuch a configuration, one or more additional rows of the detectorelements 202 are arranged in a parallel configuration for acquiring theprojection data. FIG. 2 includes a Cartesian coordinate system, and thedetector elements 202 extend in row along the x axis. It is to beunderstood that additional rows of detector elements may extend alongthe z axis.

In certain embodiments, the system 200 is configured to traversedifferent angular positions around the subject 204 for acquiring desiredprojection data. Accordingly, the gantry 102 and the components mountedthereon may be configured to rotate about a center of rotation 206 foracquiring the projection data, for example, at different energy levels.Alternatively, in embodiments where a projection angle relative to thesubject 204 varies as a function of time, the mounted components may beconfigured to move along a general curve rather than along a segment ofa circle.

In one embodiment, the system 200 includes a control mechanism 208 tocontrol movement of the components such as rotation of the gantry 102and the operation of the x-ray radiation source 104. In certainembodiments, the control mechanism 208 further includes an x-raycontroller 210 configured to provide power and timing signals to theradiation source 104. Additionally, the control mechanism 208 includes agantry motor controller 212 configured to control a rotational speedand/or position of the gantry 102 based on imaging requirements.

In certain embodiments, the control mechanism 208 further includes adata acquisition system (DAS) 214 configured to sample analog datareceived from the detector elements 202 and convert the analog data todigital signals for subsequent processing. The data sampled anddigitized by the DAS 214 is transmitted to a computing device (alsoreferred to as processor) 216. In one example, the computing device 216stores the data in a storage device 218. The storage device 218, forexample, may include a hard disk drive, a floppy disk drive, a compactdisk-read/write (CD-R/W) drive, a Digital Versatile Disc (DVD) drive, aflash drive, and/or a solid-state storage device.

Additionally, the computing device 216 provides commands and parametersto one or more of the DAS 214, the x-ray controller 210, and the gantrymotor controller 212 for controlling system operations such as dataacquisition and/or processing. In certain embodiments, the computingdevice 216 controls system operations based on operator input. Thecomputing device 216 receives the operator input, for example, includingcommands and/or scanning parameters via an operator console 220operatively coupled to the computing device 216. The operator console220 may include a keyboard (not shown) or a touchscreen to allow theoperator to specify the commands and/or scanning parameters.

Although FIG. 2 illustrates only one operator console 220, more than oneoperator console may be coupled to the system 200, for example, forinputting or outputting system parameters, requesting examinations,and/or viewing images. Further, in certain embodiments, the system 200may be coupled to multiple displays, printers, workstations, and/orsimilar devices located either locally or remotely, for example, withinan institution or hospital, or in an entirely different location via oneor more configurable wired and/or wireless networks such as the Internetand/or virtual private networks.

In one embodiment, for example, the system 200 either includes, or iscoupled to a picture archiving and communications system (PACS) 224. Inan exemplary implementation, the PACS 224 is further coupled to a remotesystem such as a radiology department information system, hospitalinformation system, and/or to an internal or external network (notshown) to allow operators at different locations to supply commands andparameters and/or gain access to the image data.

The computing device 216 uses the operator-supplied and/orsystem-defined commands and parameters to operate a table motorcontroller 226, which in turn, may control a motorized table 228.Particularly, the table motor controller 226 moves the table 228 forappropriately positioning the subject 204 in the gantry 102 foracquiring projection data corresponding to the target volume of thesubject 204.

As previously noted, the DAS 214 samples and digitizes the projectiondata acquired by the detector elements 202. Subsequently, an imagereconstructor 230 uses the sampled and digitized x-ray data to performhigh-speed reconstruction. Although FIG. 2 illustrates the imagereconstructor 230 as a separate entity, in certain embodiments, theimage reconstructor 230 may form part of the computing device 216.Alternatively, the image reconstructor 230 may be absent from the system200 and instead the computing device 216 may perform one or morefunctions of the image reconstructor 230. Moreover, the imagereconstructor 230 may be located locally or remotely, and may beoperatively connected to the system 100 using a wired or wirelessnetwork. Particularly, one exemplary embodiment may use computingresources in a “cloud” network cluster for the image reconstructor 230.

In one embodiment, the image reconstructor 230 stores the imagesreconstructed in the storage device 218. Alternatively, the imagereconstructor 230 transmits the reconstructed images to the computingdevice 216 for generating useful patient information for diagnosis andevaluation. In certain embodiments, the computing device 216 transmitsthe reconstructed images and/or the patient information to a display 232communicatively coupled to the computing device 216 and/or the imagereconstructor 230.

In certain embodiments, the system 200 may include a pre-patientcollimator plate 236 positioned in between the x-ray radiation source104 and the subject or patient 204 for reducing patient x-ray doseduring a scan with the system 200. The pre-patient collimator plate 236includes an aperture that shapes the x-ray beam 106 reaching the subject204 by adjusting a width of the beam leaving the x-ray tube, forexample. Herein, by adjusting the aperture width, the width of the beammay be adjusted. The pre-patient collimator plate 236 adjusts the widthof the beam to match the size of the detector array 108 so that anyunnecessary patient dose is reduced.

X-ray radiation that impinges on the subject 204 is attenuated as ittraverses the subject 204. The radiation that traverses the subject 204is detected by one or more pixels, or channels of the detector array 108and a signal is generated that is indicative of characteristics of theradiation that is detected by the pixel. A CT image may be reconstructedfrom the signal. In addition, some of the radiation impinging on thesubject 204 may be scattered (e.g., due to interactions with the subject206). While the unscattered attenuated radiation is referred to asprimary radiation, the scattered radiation is referred to as secondaryradiation. As such, the secondary radiation that is detected by the oneor more pixels of the detector array 108 may increases noise and furtherreduce the quality of the CT image produced based upon the detectorsignal.

In order to reduce the scattered or secondary radiation from beingdetected by the detector array 108, the system 200 may include ananti-scatter collimator or post-patient collimator 234 positionedbetween the subject 204 and the detector array 108. The post-patientcollimator 234 is configured to allow the primary radiation to passthrough the collimator to be detected by the pixel of the detector array108. In addition, the post-patient collimator 234 blocks the scatteredradiation from reaching the detector array 108. As such, thepost-patient collimator 234 may absorb the scattered radiation therebyshielding the detector array 108 from the scattered radiation, and thusreducing noise in the pixels of the detector array 108, as shown below.The post-patient collimator may include a plurality of collimatingelements (also referred to as high-attenuating regions) that extend in arow along the x axis with the detector elements 202. Further, while notvisible in FIG. 2, each collimator element may extend along an entiretyof the detector array in the z axis. In other examples, the plurality ofcollimator elements may be arranged in multiple rows and thus multiplecollimator elements may extend along the z axis.

Turning now to FIG. 3A, a magnified view 300 of a portion of apost-patient collimator or anti-scatter collimator 302 is shown. Thepost-patient collimator 302 is one non-limiting example of thepost-patient collimator 234 shown in FIG. 2. As described previously,the post-patient collimator 302 is positioned between a source side 312and a detector side 314 and is used to collimate the attenuated signalsarising from a subject placed in the source side 312 towards thedetector side 314. Herein, the source side 312 may include a source(such as the source 104 of FIGS. 1 and 2), a pre-patient collimator(such as the pre-patient collimator 236 of FIG. 2), and a subject (suchas the subject 204 of FIG. 2). The detector side 314 may include adetector array (such as the detector array 108 of FIGS. 1 and 2).Specifically, the post-patient collimator 302 is placed in front of thedetector array to absorb the scattered radiation and allow the primaryradiation to pass through the post-patient collimator 302 to thedetector side 314.

The post-patient collimator 302 may include high attenuating plates orregions 306 composed of materials such as tungsten or lead, alternatingwith low attenuating plates or regions 308. The high attenuating regions306 may attenuate the scattered radiation to a higher degree that thelow attenuating regions 308. Together, the high attenuating regions 306and the low attenuating region 308 may form a grid-like pattern. FIG. 3Aincludes the Cartesian coordinate system, showing that the highattenuating regions 306 are spaced along the x axis, similar to thedetector elements (not shown in FIG. 3A). Each high attenuating region306 may extend along the z axis a suitable distance (e.g., the entiretyof the detector array). FIG. 3A may be a side view of the collimator(e.g., a head-on view while looking down the bore of the CT gantry). Inother examples, FIG. 3A may be a cross-sectional view taken along thelength of the collimator at a suitable plane of the z axis.

In some examples, the regions 308 may not be composed of any material(e.g., may be void of material), so air filling the regions 308 betweenthe regions 306 may allow the radiation to pass through without anyattenuation. As such, the high attenuation regions may absorb radiationscattered from the subject, whereas the low attenuation regions mayallow the primary radiation passing through the subject to transmitthrough to channels (or pixels) of the detector (as indicated by arrows310). Said another way, the high attenuation regions allow a lowerpercentage of scattered radiation to pass through by blocking a majorportion of the scattered radiation, and the low attenuation regionsallow a higher percentage of the primary radiation to pass through. Insome example, the high attenuation regions 306 block 90% of thescattered radiation, and the low attenuation regions 308 may transmit99.9% of the primary radiation arising from the object to reach theactive regions of the detector.

The high attenuation regions 306 typically have a rectangular shape ofuniform height, h (along the y axis) and uniform width, w (along the xaxis). Each high attenuation region 306 is separated from an adjacenthigh attenuation region 306 by a space or gap, d. As such, the space dbetween adjacent high attenuation regions may correspond to the width ofthe low attenuation regions 308, for example.

The space d between each adjacent high attenuation regions 306 isuniform and may be sized to match each channel or pixel of the detector,for example. In some embodiments, the space d may be sized to match awidth W (or surface area) of the channel of the detector. Specifically,the width W of the channel represents an active region of the detectorwhere x-ray radiation is captured, and the space d may be adjusted tomatch the width of the active region of the detector. Herein, the widthw of the high attenuation regions 306 may be sized to match the inactive(or dead space or dead zone) of each channel of the detector. Forexample, each detector includes a plurality of channels including anactive region and an inactive region. Radiation detected in the activeregion of the channel is used for creating the CT images, whileradiation falling on the inactive region of the channel is not detectedby the detector. In some examples, a transverse dimension of the activeregion may be between 0.5 mm to 1.5 mm, and the transverse dimension ofthe inactive region may be between 0.2 mm to 0.3 mm. By aligning thehigh attenuation regions 306 with the inactive regions of the channeland the low attenuating regions 308 with the active regions of thechannel, scattered radiation may be blocked while the primary radiationmay be allowed to pass through to the active regions of the channel.

The rectangular shape of the high attenuation region 306 may give riseto a rectangular transition profile of the post-patient collimator 302,which in turn leads to a response of the detector having a rectangularprofile, as shown in FIG. 4. The post-patient collimator 302 having arectangular transition profile may henceforth be referred to as arectangular post-patient collimator.

Turning now to FIG. 4A, graph 400 shows a response of a detector when arectangular post-patient collimator (such as the rectangularpost-patient collimator 302 of FIG. 3) is placed in front of thedetector. The x-axis of graph 400 represents a transverse distanceacross a channel of the detector and the y-axis represents a spatialdomain response of the detector. The spatial domain response of thedetector is a measure of an amount of radiation detected across aspatial region formed by a channel of the detector.

When the rectangular post-patient collimator as described with referenceto FIG. 3A is aligned with individual channels (specifically, individualactive regions of the detector, for example) of the detector, the x-raybeam may be able to transmit only through the low attenuation regions308 of the collimator and be detected by the channels of the detector.Thus, the response of the detector channel may include a rectangularshape as shown by response 402 in graph 400. Herein, the width of theresponse 402 may be equal to the space d between adjacent highattenuation regions. Specifically, the width of the response 402 may beequal to the width of the low attenuation region 308 shown in FIG. 3A.

The y-axis of response 402 represents the spatial domain response of thedetector. For example, the spatial domain response of the detector is ameasure of a sum total radiation detected by all the channels of thedetector. For example, the spatial domain response (R) of the detectormay be mathematically represented by equation (1) as shown below:R=Σ _(k=0) ^(n) R _(k)  (1)

where R_(k) is the response of k^(th) channel of the detector. Herein,the detector includes n number of channels and R_(k) is a measure of theamount of radiation detected by the kth channel.

Mathematically, the rectangular response R of the detector may berepresented in 1-D by equation (2), as shown below:

$\begin{matrix}{{R(r)} = \left\{ \begin{matrix}{{R\;\max},} & {{{r} < \frac{X}{2}},} \\{0,} & {{r} \geq {\frac{X}{2}.}}\end{matrix} \right.} & (2)\end{matrix}$where Rmax is the maximum response of the detector, r is the distancealong the detector, and X is equal to the width of the detector (orrectangular function).

In the example detector response shown in graph 400, R(x)=0 when r≥0.55and R(x)=Rmax when |r|<0.55. Thus, the detector detects a maximum amountof primary radiation in the region between −0.55 mm and +0.55 mm.Herein, the width of the response is equal to 1.10 mm. A sharptransition occurs at the boundaries occurring at −0.55 mm and +0.55 mm.For example, all of the radiation is blocked at r<−0.55, and theresponse R=0, however, at r=−0.55, the response R quickly reaches Rmax,indicating that a maximum amount of radiation is getting through to thedetector. Likewise, another sharp transition occurs at r=0.55. Thesesharp transitions may lead to high frequency side lobes in the frequencydomain as shown below.

For a rectangular spatial domain response of the detector as shown byresponse 402, a corresponding frequency domain response may be computedby performing a 1-D Fourier Transform of the spatial domain response.The Fourier Transform is a signal processing tool which is used todecompose a signal into its sine and cosine components. The output ofthe transformation represents the signal in the Fourier or frequencydomain, while the input image is the spatial domain equivalent. In theFourier domain image, each point represents a particular frequencycontained in the spatial domain image. Mathematically, the frequencydomain response may be represented by equation (3) as shown below:R(u)=∫_(−∞) ^(∞) R(r)e ^(−j2πur) dr=Xsinc(πXu)  (3)

The graphical representation of the sinc function is shown in graph 500of FIG. 5. Turning to FIG. 5, a frequency domain response 501 includes acentral frequency response 502 (shown as dashed line), and additionalhigher frequency components 504 (dashed lines). The frequency response502 of the detector represents the signal that is used to generate animage of the object that is being scanned. However, the higher frequencycomponents 504 contribute to noise in the imaging system. Herein, thehigh frequency components 504 are generated because of the sharptransition occurring at the boundary of the rectangular responsefunction, for example. As such, these high frequency components 504 maywrap around and cause image artifacts, as shown in FIG. 6.

Turning now to FIG. 6, image 600 shows an example CT image of an anatomyof a subject (such as the subject 204 of FIG. 2) scanned in an imagingsystem (such as the imaging system 200 of FIG. 2). Herein, the CT imageis reconstructed using frequencies detected by a detector of the imagingsystem. When rectangular post-patient collimators are used in theimaging system, high frequency components are generated. High frequencycomponents of frequency signals detected by a detector may wrap aroundand cause streaky or bandy artifacts as indicated by arrows 604 in theimage 600. Such artifacts, called aliasing or wrap around artifacts,interfere with a clinician's ability to read the image 600.

To combat aliasing, one or more anti-aliasing methods such as quarterdetector offset, focal spot wobble, and comb filtering may be used.Typically, over sampling may reduce aliasing artifacts in the CT image.In quarter detector offset method, the detector center is offset by aquarter of the detector cell width with respect to the iso-center of thedetector. As a result, when the gantry rotates 180°, each sample isinterleaved with a previously acquired sample, thus increasing thesampling by a factor of two. Thus, such an arrangement results in doublesampling and reduces aliasing. Further, the quarter detector offset mayresult in a detector that is no longer symmetric, potentially wastingdetector space.

In focal spot wobble method, the focal spot of the x-ray beams may beintentionally shifted or switched or wobbled between two or more optimalfocal spot locations during a scan or between scans, thereby at leastdoubling the sampling in the system. However, additional components maybe integrated in the x-ray tube to deflect (either electrostatically orelectromagnetically) the focal spot of the x-ray beam. Deflecting thefocal spot to two locations, for example, requires doubling the samplingrate to keep the same number of views per rotation. This increase insampling rate reduces the amount of signal in each acquisition,potentially increasing noise and low signal artifacts.

In comb filtering method, a diaphragm is inserted in front of thedetector to reduce an aperture of the detector to increase the spatialresolution of the detector. Herein, the diaphragm reduces the apertureopening of each pixel of the detector, which changes a modulationtransfer function of the detector to higher frequencies wherein themodulation transfer function is a spatial frequency response of thedetector. As such, the reduction in aperture increases the amount ofaliasing, which may be reduced by oversampling with multiple focal spotpositions or focal spot wobble as previously described. However, in suchsystems, the dose efficiency of the detector may be reduced. The doseefficiency is a measure of the amount of x-rays that are needed toproduce an image of desirable/suitable quality, relative to an idealdetector. If the dose efficiency decreases, a higher amount of x-raysmay be needed to produce the image with the desired quality, causingincreased radiation dose for the patient. Adding the hardware to enablethis feature may be difficult to manufacture and may be expensive.

The inventors have recognized that it may be possible to reduce aliasingin the CT images without using additional measures such as focal spotwobble, for example. Herein, the proposed method includes analogpre-filtering of the signals passing through the subject by utilizing anon-rectangular post-patient collimator as shown in FIG. 3B.

Turning now to FIG. 3B, a magnified view 350 of a portion of apost-patient collimator 352 is shown. It may be appreciated that thepost-patient collimator 352 may extend in two dimensions and may bealigned with respect to a detector array (such as detector array 108 ofFIGS. 1 and 2). The post-patient collimator 352 may be a non-limitingexample of the post-patient collimator 234 shown in FIG. 2. FIG. 3Bincludes the Cartesian coordinate system, showing that the highattenuating regions 360 are spaced along the x axis, similar to thedetector elements (not shown in FIG. 3A). Each high attenuating region360 may extend along the z axis a suitable distance (e.g., the entiretyof the detector array). FIG. 3B may be a side view of the collimator(e.g., a head-on view while looking down the bore of the CT gantry). Inother examples, FIG. 3B may be a cross-sectional view taken along thelength of the collimator at a suitable plane of the z axis.

The post-patient collimator 352 may include a shape that reducesaliasing artifacts. Specifically, the shape of the post-patientcollimator 352 may include an anti-aliasing shape so that the transitionprofile does not include a sharp transition (as described with referenceto FIGS. 3A and 4A). Herein, the anti-aliasing shape of the post-patientcollimator 352 results in a smoother or tapered transition profile asdescribed below. Hereafter, the post-patient collimator 352 may bereferred to as an anti-aliasing post-patient collimator.

Similar to the rectangular post-patient collimator 302 of FIG. 3A, theanti-aliasing post-patient collimator 352 may include a plurality ofhigh attenuation regions 360 interleaved with a plurality of lowattenuation regions 362. Each high attenuating region 360 may include anon-uniform transition profile and may further be aligned with arespective inactive region of a respective channel of a detector of themedical imaging system as described below. In contrast to therectangular post-patient collimator 302, the anti-aliasing post-patientcollimator includes a non-rectangular shape that reduces aliasing. Somenon-limiting examples of the anti-aliasing shape of the collimatorinclude a sinusoidal shape, a curved shape, a dome or bell shape, anapse, a rounded-trapezoidal shape, a rounded-pyramidal shape, and thelike. Herein, the rounded-trapezoidal shape includes a trapezoid withrounded edges and the rounded-pyramidal shape includes a pyramid withrounded vertex, for example. Additional shapes include designs based onGaussian profiles or the spatial response of known anti-aliasingfilters, either analog or digital, such as the Kaiser window filter.Each high-attenuating region may extend non-uniformly along at least oneaxis. As shown the high-attenuating regions extend non-uniformly alongthe y axis and along the x axis. Each high-attenuating region extenduniformly along the z axis in some examples. In other examples, eachhigh-attenuating region may extend non-uniformly along the z axis (e.g.,the front and/or back face of each high-attenuating region may becurved).

Similar to the rectangular post-patient collimator 302, theanti-aliasing post-patient collimator 352 is positioned between a sourceside 356 and a detector side 358 and is used to collimate the attenuatedsignals arising from the subject towards the detector side 358. Herein,the source side 356 may include a source (such as the source 104 ofFIGS. 1 and 2), a pre-patient collimator (such as the pre-patientcollimator 236 of FIG. 2), and a subject (such as the subject 204 ofFIG. 2). The detector side 358 may include a detector array (such as thedetector array 108 of FIGS. 1 and 2). Specifically, the anti-aliasingpost-patient collimator 352 is placed in front of the detector array toabsorb the scattered radiation and additionally modulate or shape theprimary radiation passing through the anti-aliasing post-patientcollimator 352 as described below. Said another way, the anti-aliasingpost-patient collimator may lead to a non-uniform attenuation of thesignals reaching the pixels of the detector.

Similar to the rectangular post-patient collimator 302 of FIG. 3A, theanti-aliasing post-patient collimator 352 includes a plurality of highattenuation regions 360 composed of materials attenuating the radiationby a higher degree, such as tungsten or lead, alternating with aplurality of low attenuating regions 362 composed of materialsattenuating the radiation to a lower degree. Together, the pluralitiesof regions 360 and 362 form a grid-like pattern. In some example, theplurality of regions 362 may not be composed of any material (e.g.,filled with air), thus allowing the radiation to pass them throughwithout any attenuation. In such an example, each high attenuationregion 360 may be separated from an adjacent high attenuation region bya gap.

Similar to the rectangular post-collimator 302, the high attenuationregions 360 of the anti-aliasing post-patient collimator 352 may absorbradiation scattered from the subject, while the low attenuation regions362 of the anti-aliasing collimator 352 may allow the primary radiationpassing through the subject to transmit through to channels (or pixels)of the detector (as indicated by arrows 364). Thus, the high attenuationregions allow a lower percentage of scattered radiation to pass throughby blocking a major portion of the scattered radiation, and the lowattenuation regions allow a higher percentage of the primary radiationto pass through. In contrast to the rectangular post-patient collimator302, the anti-aliasing post-patient collimator 352 may additionallyshape or modulate the primary radiation based on a shape of theanti-aliasing post-patient collimator as discussed below. In oneexample, the anti-aliasing post-patient collimator may modulate thesignals in a non-uniform manner, resulting in a reduction in aliasingartifacts as explained below.

Specifically, in contrast to the sharp transition profile of the highattenuation region 306 of the rectangular post-patient collimator 302,the high attenuation region 360 of the anti-aliasing post-patientcollimator 352 may include a smoother or gradient transition profile.For example, sides 366 of the high attenuation region have asinewave-like or a distorted sinewave-like profile or bell-shapeprofile. Herein, the transition from point A to point C of the highattenuation region 360 of the anti-aliasing post-patient collimator 352is gradient and not sharp. Likewise, the transition from point C topoint B of the anti-aliasing post-patient collimator 352 is alsogradient. Together, curves AC and CB form the transition profile of thehigh attenuation region 360 of the anti-aliasing post-patient collimator352.

Unlike the rectangular post-patient collimator 302, the width w of thehigh attenuation regions 360 of the anti-aliasing post-patientcollimator 352 is not uniform. A bottom portion of the high attenuationregion 360 may have a larger width compared to a top portion of the highattenuation region. The bottom portion of the high attenuation region360 may correspond to the region that is closer to the detector sidewhile the top portion of the high attenuation region 360 may correspondto the region closer to the source side of the imaging system. Thus, thewidth of the high attenuation region 360 of the anti-aliasingpost-patient collimator 352 increases as the distance of the collimatorto the detector decreases. In other words, the bottom most portion ofthe high attenuation region 360 has the maximum width (e.g.,AB=w_(max)), and the topmost portion of the high attenuation region 360has the minimum width, w_(min). Thus, the width of the high attenuationregion 360 is not uniform and further varies along the y-axis.

In some examples, the maximum width w_(max) may be larger than a widthof the inactive region (W_(inactive)) of a detector channel such thatthe high attenuation region may partially overlap into the active regionof the detector channel. An extent of overlap may be based on a shape ofthe of the high attenuation region of the anti-aliasing post-patientcollimator, for example. The collimator length (in the y direction) overthe inactive region may vary, but anti-aliasing benefits may only berealized when the collimator length is varying over the active region ofthe detector. Varying the collimator length over the inactive area mayprovide benefits if the collimator gets misaligned, for example.

In addition, a height h1 of the high attenuation region 360 of theanti-aliasing post-patient collimator 352 is not uniform. Herein, theheight h1 varies along the x-axis. At point A, the height of the highattenuation region 360 is zero. Then, the height of the high attenuationregion 360 gradually increases along the x-axis, reaching a maximumheight, H1, at point C, thereafter decreases continually along the curveCB, and finally becomes zero at point B. In this way, the highattenuation region 360 includes a smoother transition profile.

Similar to the high attenuation region 360, the low attenuation region362 may include a height h2 that is not uniform. The height h2 variesalong the x-axis. At point C, the height of the low attenuating region362 is zero. The height h2 gradually increases along the x-axis frompoint C, reaching a maximum height H2 at point B. Between points B andE, the height of the low attenuating regions remains maximum at H2. Frompoint E to point F, the height h2 of the low attenuating region 362continues to decrease, and finally reaches zero at point F. In this way,the low attenuation region 362 includes a smoother transition profile.

Consider an example configuration wherein the low attenuation regionsare void of materials. In such an example, adjacent high attenuationregions 360 of the anti-aliasing post-patient collimator 352 areseparated by a gap or space, d(y). Unlike the space d between adjacenthigh attenuation regions of the rectangular post-patient collimator 302,the gap d(y) between adjacent high attenuation regions of theanti-aliasing post-patient collimator 352 is not uniform. The gap d(y)between adjacent high attenuation regions may correspond to the width ofthe low attenuation regions 362, for example. Thus, the width of the lowattenuation regions 362 of the anti-aliasing post-patient collimator 352is also non-uniform.

The gap d(y) (or width) of the low attenuation regions 362 is larger ator near the detector side 358, while the gap d(y) is smaller towards thesource side 356 of the imaging system. More specifically, the gap d(y)between points B and E is smaller than the gap d(y) between points C andF in view 350. Thus, the gap d(y) varies from a minimum d_(min) to amaximum d_(max) when moving along the y-axis from the detector side 358to the source side 356 of the imaging system. In some examples, d_(min)may be smaller than a width of the active region of the detectorchannel.

Thus, the varying gap between adjacent high attenuation regions and thevarying width of the high attenuation regions of the anti-aliasingpost-patient collimator 352 results the post-patient collimator 352having a varying area. In some examples, the low attenuation regions 362may have an inverted shape of the high attenuation regions 360. In theview 350, the high attenuation region 360 may include a dome or bellshape, and the low attenuation region 362 may include an inverted domeor bell or cup shape, for example. Together, the high and the lowattenuation regions of the anti-aliasing post-patient collimator 352modulate the primary radiation as indicated by arrows 364. By varyingthe gap, a pixel aperture may be modulated. This “apodization” mayresult in non-uniform attenuation of the primary radiation.

For illustrative purposes, a length of each arrow 364 represents a levelor amount of primary radiation, assuming no patient attenuation for thisexample, passing through the anti-aliasing post-patient collimator 352and reaching the detector (e.g., an active region of the detector).Herein, the width of the active region may be W_(active). In someexamples, W_(active) may be larger than W_(inactive). In some otherexamples, W_(active) may be equal to W_(inactive). For example, asmaller length of the arrow 364 indicates a lower amount of the primaryradiation that is transmitted to the detector channel. A larger lengthof the arrow 364 indicates a higher amount of primary radiation that istransmitted to the detector channel. Herein, an amount of primaryradiation passing through the high attenuation region 360 may increasewith decreasing height (and increasing width) of the high attenuationregions. Said another way, the amount of attenuation of the signals bythe region 360 is proportional to the height h1 of the region 360 (orthe amount of transmission is inversely proportional to the height ofthe region 306). In addition, the level of transmission of the signalsthrough the low attenuation region is directly proportional to theheight h2 of the low attenuation region. Thus, the central regions ofthe high attenuation regions 360 block a higher percentage of theprimary radiation than the ends or edges of the high attenuation regions360. In other words, some of the primary radiation incident along theends of the high attenuation regions leaks into the detector channel. Inthis way, the post-patient collimator 352 may filter the incomingprimary radiation. It may be appreciated that the incoming primaryradiation is attenuated by the object placed in the imaging system.Herein the non-uniform profile of the anti-aliasing post-patientcollimator is non-uniform across an entire cross-section area of thehigh attenuating region and is also non-uniform across an entirecross-section area of the low-attenuating region. In this way, thetransmission of the signal (e.g., radiation) may be non-uniform acrossthe entire cross-sectional area of the high-attenuating region and maybe non-uniform across the entire cross-sectional area of thelow-attenuating region, even if the regions re shaped with portions thatattenuate uniformly. For example, it may be noted that there may be someregions in the low attenuation region (e.g., between points B and E inview 350) where the transmission of the primary radiation may beuniform.

Thus, by incorporating the anti-aliasing shape to the post-patientcollimator, the signals undergo an analog pre-filtration in the spatialdomain. Herein, by pre-filtering (e.g., filtering performed beforesignals reach the detector) the signals in the spatial domain, higherfrequency components that would otherwise wrap around and generatealiasing artifacts in the image may be reduced, as described below.

The smoother transition profile of the anti-aliasing post-patientcollimator 352 may be achieved by using any non-rectangular overallshape. Some examples of non-rectangular shapes include bell shape, domeshape, sinusoidal shape, and the like. Some more examples of suchnon-rectangular shapes include trapezoidal shape, pyramidal shape, andthe like. The edges in these shapes may be additionally rounded toreduce sharp transitions for the signals. In some examples, a level ofattenuation of signals by the plurality of high attenuating regions maybe based on the shape of the high attenuation regions. Additionally, alevel of transmission of signals by the low attenuating regions may bebased on the shape of the high attenuation regions. Further, in someexamples an overall non-rectangular shape may be achieved by a pluralityof high-attenuating areas that are shaped together to form thenon-rectangular shape, where the high-attenuating areas themselves arerectangular, such as a plurality of small rectangles that collectivelyform a non-rectangular shape. In still further examples, thehigh-attenuating region may be comprised of materials with differentattenuating properties, where a higher-attenuating material is formed ina non-rectangular shape surrounded by lower-attenuating material. Inthis way, the shape itself may be rectangular, but the attenuation maybe non-rectangular. Additionally, an overall rectangular shape may beused if the collimator is rotated with respect to the detector array,such that a non-uniform transmission profile/response is still achieved.

Turning now to FIG. 4B, graph 450 shows a response of a detector when ananti-aliasing post-patient collimator (such as the anti-aliasingpost-patient collimator 352 of FIG. 3B) is placed in front of thedetector. The x-axis of graph 450 represents a transverse distanceacross a channel of the detector and the y-axis represents a spatialdomain response of the detector.

When the anti-aliasing post-patient collimator as described withreference to FIG. 3B is aligned with individual channels or pixels(specifically, individual active regions of the detector, for example)of the detector, the x-ray beam may be able to transmit only through thelow attenuation regions 360 of the collimator and be detected by thechannels of the detector. Due to the non-uniform width of the low andhigh attenuation regions of the anti-aliasing post-patient collimator,the spatial domain response R of the detector may not include arectangular shape. In one example, the spatial domain response R of thedetector may include a trapezoidal shape as indicated by plot 404.Herein, the width of the response 404 may not be uniform.

Mathematically, the response R of the detector may be represented in 1-Dby equation (4), as shown below:

$\begin{matrix}{{R(x)} = \left\{ \begin{matrix}{{R\;\max},} & {{d} < \frac{L\; 2}{2}} \\{0,} & {{d} > \left( {\frac{L\; 2}{2} + \delta} \right)} \\{{\left( {\frac{L\; 1}{2} - {d}} \right)*\frac{R\;\max}{\delta}},} & {else}\end{matrix} \right.} & (4)\end{matrix}$

where Rmax is the maximum response of the detector, d is the distancealong the detector, m is the slope of the sides of the response R, and

$\delta = {\frac{L_{1} - L_{2}}{2}.}$Herein, L₁ represents the bottom length of the response R and L₂represents the top length of the response R. Herein, the bottom lengthL₁ includes the length from when the responses begins to increase abovezero to when the response again reaches zero and the top length L₂includes the length where the response is equal Rmax

In the example detector response shown in graph 450, R(x)=0 when x≥0.6and R(x)=Rmax when |x|<0.45. Thus, the detector detects a maximum amountof primary radiation in the region between −0.45 mm and +0.45 mm.Herein, the maximum response Rmax occurs within 0.90 mm. In addition,the response R(x) includes two linear transitions between the high andlow response regions. Herein, the line 406 has a positive slope, whilethe line 408 includes a negative slope. The slope of the lines 406 and408 may depend on an extent of overlap of the high attenuation region360 with the active region of the detector channel, for example.

Similar to the rectangular response shown in FIG. 4A, a frequency domainresponse of the response 404 may be computed by performing a 1-D FourierTransform of the spatial domain response. Thus, by using anon-rectangular shape of the post-patient collimator, the sampling onthe detector may also be non-rectangular. The non-rectangular shape ofthe detector serves as an analog pre-filter to the spatial signal on thedetector. This pre-filtering of the signal reduces higher frequencysignals that would otherwise wraparound as aliasing as shown in FIG. 5.

Turning now to FIG. 5, graph 500 shows a frequency domain response 503of the non-rectangular response shown in FIG. 4B overlaid with thefrequency domain response 501 of the rectangular detector response shownin FIG. 3A. As seen in graph 500, the central frequency response 502remains essentially unchanged; however, the higher frequency components504 of the non-rectangular spatial domain response of FIG. 4B arereduced compared to the higher frequency components 506 of therectangular spatial domain response shown in FIG. 3A. As explainedpreviously, the high frequency components cause aliasing artifacts inthe CT images. In this way, anti-aliasing shape of the post-patientcollimator leads to a reduction in high frequency components detected bythe detector. This, in turn, feeds forward and reduces the aliasing seenin the final clinical image. In this way, aliasing artifacts in the CTimages may be reduced by incorporating an anti-aliasing shapedpost-patient collimator without using focal spot wobble and/or anypost-processing or digitization of the CT images, for example.Typically, post-processing is used to reduce aliasing. However, byperforming the post-processing on signals that are not pre-filtered,overall signal levels of the system may be reduced. By pre-filtering thesignals before they reach the detector, the overall signal levels maynot be reduced in the imaging system. It may be appreciated that theanalog pre-filtering of the signals using the anti-aliasing post-patientcollimator is performed prior to any post-processing or digitization ofthe signals, and not after the signals have been digitally sampled. Inthis way, analog pre-filtering of the signals in the spatial domainusing the anti-aliasing post-patient collimator reduces aliasingartifacts in the CT images.

The example embodiment described so far with reference to FIG. 3Bincludes incorporating the anti-aliasing shape to the post-patientcollimator of the imaging system. In another embodiment, it may bepossible to include the anti-aliasing shape in a collimator platecoupled to an anti-scatter leaf, as shown in FIG. 7.

Turning now to FIG. 7, a schematic view 700 of a detector 702 of animaging system is shown. The detector 702 may be one non-limitingexample of the detector array 108 shown in FIGS. 1 and 2 of the imagingsystem such as the imaging system 200 of FIG. 2. As described above, CTimaging systems include an x-ray source (such as the source 104 of FIGS.1 and 2) that emits a fan-shaped beam toward a subject or object (suchas the subject 204 of FIG. 2). The beam, after being attenuated by thesubject, impinges upon an array of radiation detectors. The intensity ofthe attenuated beam radiation received at the detector array istypically dependent upon the attenuation of the x-ray beam by thesubject. Herein, each detector element of the detector array produces aseparate electrical signal indicative of the attenuated beam received byeach detector element. The electrical signals are processed to generatea CT image of the subj ect.

The CT imaging system may include a collimator 704, scintillator cells710, a reflector channel 712, and a photodiode (not shown in FIG. 7).Herein, the collimator 704 collimates the x-ray beams received at thedetector 702 and the scintillator cell 710 converts the x-ray to lightenergies, and the photodiode receives the light energy from thescintillator and converts the light energy into electrical signals. Assuch, the output of the photodiodes is transmitted to image processingsystems (such as image processing system 10 shown in FIG. 1) for imageconstruction.

The collimator 704 includes an anti-scatter leaf 706 coupled to acollimator plate 708. Together, the anti-scatter leaf 706 and thecollimator plate 708 absorb the scattered radiation and reduce noise inthe system. In some examples, the collimator 704 may be composed ofhighly absorbing materials such as tungsten or lead. In other examples,the collimator 704 may be composed of aluminum. Herein, the anti-scatterleaf 706 and the collimator plate 708 may be composed of the samematerial, or different material.

FIG. 7 includes the Cartesian coordinate system, showing that thescintillator cells are spaced along the x axis. The leaf 706 extendsalong they axis (e.g., has a longitudinal axis parallel to the y axis).FIG. 7 may be a side view of the collimator, leaf, and detector (e.g., ahead-on view while looking down the bore of the CT gantry). In otherexamples, FIG. 7 may be a cross-sectional view taken along the length ofthe collimator, leaf, and detector at a suitable plane of the z axis.

The collimator plate 708 may be aligned with the reflector channel 712disposed between adjacent scintillator cells. In some examples, a widthWr of the reflector channel 712 between the scintillator cells mayrepresent an inactive region, and a width Ws of the scintillator cellmay represent an active region. The reflector material reduces lightleaking into adjacent scintillator cells. The inventors have recognizedthat it may be possible to include an anti-aliasing shape to thecollimator plate 708 to reducing aliasing artifacts in the CT images. Asexplained previously with reference to FIG. 3B, the anti-aliasing shapemay modulate the x-ray beam reaching the scintillator 710, and thusreduce aliasing artifacts by pre-filtering the signals.

The anti-scatter leaf 706 may include a rectangular shape of length L3and height H2 with H2>L3, where the leaf 706 extends uniformly in eachaxis of the coordinate system. A bottom of the anti-scatter leaf may becoupled to a top of the collimator plate 708. A magnified view 800 ofthe collimator plate 708 is shown in FIG. 8A. Tuning now to FIG. 8A, thecollimator plate 708 has a bottom 804 of length L1, and a top 802 oflength L2, parallel to each other, and separated by a thickness H1. Inone example, the collimator plate 708 may be composed of aluminum. Insuch an example, for a monoenergetic beam of 80 keV, the thickness H1 ofthe collimator plate 708 may be 3 mm. The length L1 of the bottom 804may be larger than the length L2 of the top 802. In one specificexample, the length L1 may be 0.28 mm and the length L2 may be 0.15 mm.However, the length L2 may be larger than the length L3 of theanti-scatter leaf 706 shown in FIG. 7. Thus, L1>L2>L3.

The collimator plate 708 may include a first side 806 and a second side808. The first and the second sides may not be straight, but may have anarc shape. In one example, the radius of curvature of both the first andthe second sides may be the same. In another example, the radius ofcurvature of the first and the second sides may be different. Herein, abottom of the first side 806 is separated at a distance L1 from a bottomof the second side 808, while a top of the first side 806 is at adistance L2 from a top of the second side 808. Together, the top 802,the bottom 804, the first side 806, and the second side 808 may form a“pi” shaped collimator plate 708. Hereafter, the collimator plate 708may be referred to as an anti-aliasing collimator plate.

Mathematically, the anti-aliasing shape of the collimator plate 708 maybe represented in 1-D by equation (5), as shown below:

$\begin{matrix}{{t(x)} = \left\{ \begin{matrix}{{H\; 1},} & {{d} < \frac{X}{2}} \\{0,} & {{d} > \left( {\frac{X}{2} + \delta} \right)} \\{\frac{- {\ln\left\lbrack {\left( {\frac{L\; 1}{2} - {d}} \right)*\frac{H\; 1}{\delta}} \right\rbrack}}{\mu},} & {else}\end{matrix} \right.} & (5)\end{matrix}$where t(x) represents the transition profile of the collimator plate, H1represents the maximum height of the plate, d represents the transversedistance in mm, r μ represents the attenuation coefficient of thecollimator plate 708 at the given energy and δ=(L1−L2)/2.

A transmission profile 850 of the collimator plate 708 is shown in FIG.8B. As shown in FIG. 8B, the transmission profile 850 includes atrapezoidal shape. The x-axis represents the thickness in mm and they-axis represents the transmission of the x-rays through the collimatorplate 708. Herein, a transmission of 1 indicates 100% transmission,meaning all the x-ray beams are transmitted without any attenuation. Atransmission of 0 indicates 0% transmission, implying that all the x-raybeams are blocked. A transmission of 0.5 indicated 50% transmission,implying that 50% of the incoming x-rays are blocked at the collimatorplater, while a remaining 50% is transmitted through to thescintillator.

Due to the anti-aliasing shape of the collimator plate 708, thetransmission profile of the collimator plate includes a trapezoidalshape. Herein, the arc shape of the first and the second sides of thecollimator plate 708 allow a portion of the x-rays to transmit throughwhile blocking a remaining portion of the x-rays. As the thickness ofthe first side and the second side increase, the amount of transmissionthrough the sides decreases. At the full width half maximum of thecollimator plate 708, about 50% of the x-ray beams are transmitted tothe scintillator. In this way, the anti-aliasing shape of the collimatorplate 708 modulates the signals passing through them. Consequently,higher frequency components of the signals are reduced as shown in FIG.9. Before turning to FIG. 9, it may be noted that in some examples, theanti-aliasing shape may be incorporated to the scintillator material.For example, the scintillator material forming scintillator 710 mayinclude a shape (such as an anti-aliasing shape as explained previously)that modulates the signals directly and filters the signals to reducethe high frequency components. This is in contrast to the collimatorplate providing the modulation in previous examples. In this example,the scintillator material would be thinner at the edges of the cell andreach full thickness toward the center of the cell.

Turning to FIG. 9, a modulation transfer function (MTF) 900 of adetector having anti-aliasing collimator plates such as theanti-aliasing collimator plates 708 discussed in FIGS. 7, 8A, and 8B isshown. The MTF is used to describe the high contrast resolutionperformance of the detector. The MTF is a measure of the transfer ofmodulation from the subject to the image. The MTF 900 includes an MTF902 of a first detector without an anti-aliasing collimator plate (shownin dashed lines), overlaid on an MTF 904 of a second detector having ananti-aliasing collimator plate (such as the anti-aliasing collimatorplate 708 described in FIGS. 7 and 8A, shown in solid lines). Thecentral response 906 of both the first and the second detectors remainessentially the same. However, there is a reduction in the side lobeareas (compare side lobe 908 of first detector with side lobe 910 ofsecond detector). These side lobes cause spurious resolution and afterdigital sampling can wraparound to form aliasing artifacts, as well.Specifically, the second detector having the anti-aliasing collimatorplate includes a 21.5% reduction in the side lobe area compared to thefirst detector without the anti-aliasing collimator. By reducing theside lobe area in the MTF, aliasing artifacts in the final CT images maybe reduced. In this way, the anti-aliasing shape of the post-patientcollimator plates reduces aliasing artifacts in the CT images. Aliasingartifacts in the CT images may be reduced by incorporating anant-aliasing shape to the collimator plates without using additionalmethods such as focal spot wobble and/or any post-processing ordigitization of the CT images, for example. Thus, analog pre-filteringof the signals may be achieved using the anti-aliasing collimator plate.

FIGS. 1, 2, 3, 4, 7, and 8A show example configurations with relativepositioning of the various components. If shown directly contacting eachother, or directly coupled, then such elements may be referred to asdirectly contacting or directly coupled, respectively, at least in oneexample. Similarly, elements shown contiguous or adjacent to one anothermay be contiguous or adjacent to each other, respectively, at least inone example. As an example, components laying in face-sharing contactwith each other may be referred to as in face-sharing contact. Asanother example, elements positioned apart from each other with only aspace there-between and no other components may be referred to as such,in at least one example. As yet another example, elements shownabove/below one another, at opposite sides to one another, or to theleft/right of one another may be referred to as such, relative to oneanother. Further, as shown in the figures, a topmost element or point ofelement may be referred to as a “top” of the component and a bottommostelement or point of the element may be referred to as a “bottom” of thecomponent, in at least one example. As used herein, top/bottom,upper/lower, above/below, may be relative to a vertical axis of thefigures and used to describe positioning of elements of the figuresrelative to one another. As such, elements shown above other elementsare positioned vertically above the other elements, in one example. Asyet another example, shapes of the elements depicted within the figuresmay be referred to as having those shapes (e.g., such as being circular,straight, planar, curved, rounded, chamfered, angled, or the like).Further, elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example.

Turning now to FIG. 10, an example method 1000 for reducing aliasingartifacts in an imaging system is shown. Instructions for carrying outmethod 1000 herein may be executed by a processor (e.g., processor orcomputing device 216 of FIG. 2 and/or image processing unit 10 ofFIG. 1) based on instructions stored on a memory of the processor and inconjunction with signals received from sensors of the imaging system,such as the sensors described above with reference to FIGS. 1-9. Theprocessor may employ actuators of the CT imaging system to adjust theoperation of the imaging system, the collimators, and the detectors,according to the methods described below.

Method 1000 begins at 1005 by initializing a scan sequence. In someexamples, initializing the scan sequence may include powering up anx-ray source (such as the source 104 of FIGS. 1 and 2) and a detectorarray (such as detector array 108 of FIGS. 1 and 2) of an imaging system(such as imaging system 200 of FIG. 2). In addition, the processor maymove a table on which a patient is lying in between the source and thedetector array. Specifically, the controller may position of an anatomyof interest between the source and the collimator.

At 1010, the processor may detect pre-filtered analog signals at thedetector array. In one example, post-patient collimators used in theimaging system may include an anti-aliasing shape, as explained withreference to FIG. 3B. Therein, the collimator includes a non-rectangulartransition profile because of the curved shape of the collimator. As aresult, the incoming primary radiation signals that are attenuated bythe patient undergo amplitude modulation at the post-patient collimator,and high frequency components of the incoming signals are reduced. Thus,the analog signals are pre-filtered at the post-patient collimatorhaving the anti-aliasing shape. It may be appreciated that the signal isfiltered by passing the primary signal attenuated by the patient throughthe collimator. Herein, the collimator includes a first high attenuatingregions and a second, low attenuating region, wherein the first regionattenuates the primary signal by a higher amount than the second region.

Alternatively, the pre-filtering may be achieved by using collimatorplates that have an anti-aliasing shape as shown in FIG. 7.Specifically, the collimator plates placed in contact with reflectormaterial of the detector array may include a “pi” shape. As such, the“pi” shape of the collimator plate may modulate the radiation signalsand pre-filter the signals reaching the detector, for example. Herein,the non-uniform profile of the collimator plate results in non-uniformattenuation of signals passing through the collimator plate. In the “pi”shape of the collimator plate, the center region may transmit uniformly,but non-uniformly across the entire width of the plate. It may beappreciated that the signal is filtered by passing the signal throughthe collimator having a collimator plate. The filtered signal may begenerated by passing the primary signal through the collimator plate andto a scintillator of an assembly of the detector, wherein the collimatorplate may include a non-rectangular shape with at least two curvedsides.

At 1015, method 1000 includes receiving electrical signals from thephotodiodes of the detector. As explained previously, scintillators maybe included in the detector assembly to convert incoming x-ray radiationto light energies. Photo diodes then receive the light energies andgenerate electrical signals. The processor may receive the electricalsignals at 1015.

Next at 1020, method 1000 includes generating CT images with reducedaliasing artifacts. As explained previously, aliasing in the final CTimage is a common clinical problem. The aliasing artifacts are generallymore pronounced in small joint (wrist, ankle/foot, knee), C-Spine,facial bones, and inner auditory canal scans. Since the detector cell isthe main system limiter of resolution when imaging near the scaniso-center, if post-patient collimators with rectangular profiles areused, large aliasing side lobes occur in the frequency domain. However,by incorporating anti-aliasing collimators, aliasing at the detectorduring acquisition may be reduced, and thus aliasing artifacts in theimages are reduced. Additionally, low levels of aliasing that are belowthe visually detectable threshold serve to increase the “structured”noise level of the image, so an aliasing reduction may lead to noisereductions in the system.

At 1025, method 1000 includes performing post-processing on the imagesand displaying the post-processed images. In some examples, the imagesmay be digitized and post-processed. Some non-limiting examples ofpost-processing methods include two-dimensional multiplanar reformatting(MPR), volume rendering techniques such as virtual colonoscopy andtissue transition projection, and 3D rendering techniques such as shadedsurface display and maximum intensity projection (MIP). Method 1000ends.

Thus, the collimator, collimator plate, and methods described hereinprovide for a non-uniform attenuation of a source signal (e.g.,radiation) at high-attenuating regions of a collimator or collimatorplate. The non-uniform attenuation is achieved by not only interspersingof low-attenuating and high-attenuating regions, but also by the shape,material density, and/or orientation of the high-attenuating regions ofcollimator or collimator plates with respect to a detector array. Thehigh-attenuation regions may be shaped, comprised of material, ororiented in such a manner that non-uniform attenuation of the sourcesignal is provided to the detector (e.g., the high-attenuating regionsprovide a non-uniform, non-rectangular transmission profile of thesource signal to the detector). In one example, the high-attenuatingregions may have a non-uniform shape (e.g., extend non-uniformly alongat least one axis), such as the collimator illustrated in FIG. 3B,therein providing both a non-rectangular transition profile andtransmission profile. In other examples, the high-attenuating regionsmay be have non-uniform density, such that a non-rectangulartransmission profile is provided, even if the high-attenuating regionsare rectangular in overall shape. Further, the high-attenuating regionsmay be rotated with the respect to the detector array, such that atleast in cross-section, the high-attenuating regions arenon-rectangular. In this way, the anti-aliasing post-patient collimatorand anti-aliasing collimator plates may serve as analog pre-filters thatreduce aliasing artifacts without performing adjustments such asadjusting a focal spot of the source or adjusting a shape of thedetector cell, for example. In some examples, the collimator and/orcollimator plates described herein may be manufactured using an additiveprocess.

A technical effect of the disclosure is that the analog pre-filtering ofthe signals is achieved in the continuous, spatial domain, not after thesignals have been digitized.

The systems and methods described above also provide for a method for animaging system, the method comprising receiving a filtered signalgenerated by passing a primary signal through a collimator having anon-rectangular transition profile, the collimator positioned between anobject and a detector of the imaging system, and generating an imagebased on the filtered signal received at the detector. In a firstexample of the method, the method may additionally or alternativelyinclude wherein the primary signal including radiation from a source ofthe imaging system attenuated by the object placed between the sourceand the detector, and wherein receiving the filtered signal generated bypassing the primary signal through the collimator comprises receivingthe filtered signal generated by passing the primary signal through eachof a first region and a second region of the collimator, the firstregion attenuating the primary signal by a higher amount than the secondregion. A second example of the method optionally includes the firstexample, and further includes wherein a level of transmission of theprimary signal through the collimator is based on a shape of the firstregion, the shape being non-rectangular, thereby to reduce aliasingartifacts in the image. A third example of the method optionallyincludes one or more of the first and the second examples, and furtherincludes wherein a level of transmission of the primary signal throughthe first region is approximately inversely proportional to a firstheight of the first region and a level of transmission of the primarysignal through the second region is directly proportional to a secondheight of the second region. As used herein, the level of transmissionof the primary signal through the first region being approximatelyinversely proportional to the first height of the first region mayinclude the level of transmission for a given location of the firstregion being a rough first order approximation. The level oftransmission is fully described by the equation:

attenuation  (E) = ∫_(E)^(x)φ(E)exp [−μ(E)l]Where mu is the linear attenuation coefficient (which changes withenergy and material), phi is the normalized spectrum, and 1 is thelength.

A fourth example of the method optionally includes one or more of thefirst through the third examples, and further includes wherein thenon-rectangular transition profile of the collimator generates atrapezoidal shaped detector response. A fifth example of the methodoptionally includes one or more of the first through the fourthexamples, and further includes receiving the filtered signal prior toperforming one or more of post-filtering and digitization of the primarysignal. A sixth example of the method optionally includes one or more ofthe first through the fifth examples, and further includes wherein thepost-filtering includes one or more of two-dimensional multiplanarreformatting (MPR), virtual colonoscopy, tissue transition projection,shaded surface display, and maximum intensity projection, and whereinthe collimator includes scatter rejection properties.

The systems and methods described above also provide for a collimatorfor a medical imaging system, the collimator comprising a plurality ofhigh attenuating regions interleaved with a plurality of low attenuatingregions, each high attenuating region having a non-uniform transitionprofile and configured to be aligned with a respective inactive regionof a respective channel of a detector of the medical imaging system. Ina first example of the collimator, the collimator may additionally oralternatively include wherein each region of the plurality of highattenuating regions includes a first, non-rectangular shape. A secondexample of the collimator optionally includes the first example andfurther includes wherein the plurality of low attenuating regions arevoid of material. A third example of the collimator optionally includesone or more of the first and the second examples, and further includeswherein the first shape includes one or more of a bell shape, a domeshape, and a sinusoidal shape. A fourth example of the collimatoroptionally includes one or more of the first through the third examples,and further includes wherein a level of attenuation of signals in themedical imaging system by the plurality of high attenuating regions isbased on one the first shape. A fifth example of the collimatoroptionally includes one or more of the first through the fourthexamples, and further includes wherein a level of transmission ofsignals in the medical imaging system by the low attenuating regions isbased on the first shape. A sixth example of the collimator optionallyincludes one or more of the first through the fifth examples, andfurther includes wherein each region of the plurality of low attenuatingregions is aligned with a respective active region of a respectivechannel of the detector. A seventh example of the collimator optionallyincludes one or more of the first through the sixth examples, andfurther includes wherein each region of the plurality of highattenuation regions is composed of one or more of tungsten and lead.

The systems and methods described above also provide for a system, thesystem, comprising an x-ray source configured to project a beam ofx-rays towards a patient, an array of detectors configured to receive anattenuated beam passing through the patient, and a collimator insertedbetween the patient and the array of detectors and configured tonon-uniformly attenuate the beam, the collimator having bell shapedplates each separated by a gap. In a first example of the system, thesystem may additionally or alternatively include wherein the array ofdetectors is configured to receive the beam passing through thecollimator and generate a signal based on an amount of beam reaching thearray of detectors, the amount based on a curvature of the bell shapedplates of the collimator. A second example of the system optionallyincludes one or more of the first and the second example, and furtherincludes a processor configured with instructions in non-transitorymemory that when executed cause the processor to: generate an imagebased on the signal, the signal being filtered by the bell shaped platesof the collimator. A third example of the system optionally includes oneor more of the first and the second examples, and further includeswherein the amount of the beam reaching the array of detector isinversely proportional to a height of the bell shaped plates and whereinthe amount of beam passing through the gap is higher than the amount ofbeam passing through the plurality of bell shaped plates. A fourthexample of the system optionally includes one or more of the firstthrough the third examples, and further includes wherein the bell shapedplates are composed of one or more of tungsten and lead.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

As used herein, the phrase “pixel” also includes embodiments of theinvention where the data is represented by a “voxel.” Thus, both theterms “pixel” and “voxel” may be used interchangeably herein.

Also as used herein, the phrase “reconstructing an image” is notintended to exclude embodiments of the present invention in which datarepresenting an image is generated, but a viewable image is not.Therefore, as used herein, the term “image” broadly refers to bothviewable images and data representing a viewable image. However, manyembodiments generate (or are configured to generate) at least oneviewable image.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A method for an imaging system, comprising:providing a source that emits radiation, a detector, and a collimatorhaving bell shaped plates each separated by a gap that provides thecollimator with a non-rectangular curved transition profile including anon-uniform width and a non-uniform height; receiving at the detector afiltered and modulated signal generated by passing a primary signalthrough the collimator having bell shaped plates each separated by thegap that provides the collimator with the non-rectangular curvedtransition profile including the non-uniform width and the non-uniformheight, the collimator positioned between an object and the detector;and generating an image based on the filtered and modulated signalreceived at the detector.
 2. The method of claim 1, wherein the primarysignal includes radiation from the source of the imaging systemattenuated by the object placed between the source and the detector,wherein receiving the filtered and modulated signal generated by passingthe primary signal through the collimator comprises receiving thefiltered and modulated signal generated by passing the primary signalthrough each of a first region and a second region of the collimator,wherein the first region attenuates the primary signal by a higheramount than the second region, and wherein the filtered and modulatedsignal is non-uniform.
 3. The method of claim 1, wherein thenon-rectangular transition profile of the collimator generates atrapezoidal shaped detector response.
 4. The method of claim 1, furthercomprising receiving the filtered signal prior to performing one or moreof post-filtering and digitization of the primary signal.
 5. The methodof claim 2, wherein a level of transmission of the primary signalthrough the collimator is based on a shape of the first region, theshape being non-rectangular, thereby to reduce aliasing artifacts in theimage.
 6. The method of claim 2, wherein a level of transmission of theprimary signal through the first region is inversely proportional to afirst height of the first region and a level of transmission of theprimary signal through the second region is directly proportional to asecond height of the second region.
 7. The method of claim 4, whereinthe post-filtering includes one or more of two-dimensional multiplanarreformatting (MPR), virtual colonoscopy, tissue transition projection,shaded surface display, and maximum intensity projection, and whereinthe collimator includes scatter rejection properties.
 8. A medicalimaging system, comprising: an x-ray source configured to project a beamof x-rays towards a patient; an array of detectors configured to receivean attenuated beam passing through the patient; and a collimatorinserted between the patient and the array of detectors and configuredto non-uniformly attenuate and modulate the beam, the collimator havingbell shaped plates each separated by a gap; wherein the bell shapedplates each separated by a gap provides the collimator with anon-rectangular curved transition profile.
 9. The medical imaging systemof claim 8, wherein the array of detectors is configured to receive themodulated beam passing through the collimator and generate a signalbased on an amount of beam reaching the array of detectors, the amountbased on a curvature of the bell shaped plates of the collimator. 10.The medical imaging system of claim 8, wherein the bell shaped platesare composed of one or more of tungsten and lead.
 11. The medicalimaging system of claim 9, further comprising a processor configuredwith instructions in non-transitory memory that when executed cause theprocessor to: generate an image based on the signal, the signal beingfiltered and modulated by the bell shaped plates of the collimator. 12.The medical imaging system of claim 9, wherein the amount of the beamreaching the array of detector is inversely proportional to a height ofthe bell shaped plates and wherein the amount of beam passing throughthe gap is higher than the amount of beam passing through the bellshaped plates.